This research deals with a seismic retrofitting technique of installing CES portal frames to an existing RC frame building. One of characteristics in the retrofitting technique is to employ the precast construction method that precast CES members are connected at the middle of the beam and columns of the portal frame. This retrofitting technique also uses both the adhesive joint method and the post-installed anchoring method to unify the existing RC frame and the installed CES frame. Static loading tests were conducted for a total of four one-bay one-story frames including one bare frame and three retrofitted frames with the experimental parameter of connection details between the existing RC frame and the installed CES frame. Dynamic loading test was also carried out to compare the dynamic behavior with the static behavior of the retrofitted frames. This paper outlines the testing and presents the effect of the installed CES portal frame on the seismic performance of the retrofitted RC frame.